The present invention relates to surface-modified pigments based on flaky substrates having improved settling and redispersing characteristics, and to the preparation and use thereof.
Coating compositions such as lacquers, paints, printing inks, etc., that contain pigments based on flaky substrates are problematical in handling in that the pigments, owing to their size and density, tend to settle out and can then become clumped together to form a very firm sediment cake. This cake is in general very difficult to redisperse. This is true in particular on storage of lathers, paints and printing inks and in the processing thereof.
As a consequence, numerous methods have been developed for solving the problem of incorporating and handling flaky pigments in coating compositions.
DE 36 27 329, EP 0 306 056 and EP 0 268 918 disclose that modified flaky substrates bearing a polymer coating or after treatment with coupling agents such as organotitanates and organosilanes show improved settling and redispersing characteristics in coating compositions.
Furthermore, redispersion can be facilitated by adding to the coating composition additives which bring about either a controlled flocculation (house of cards effect), a pseudoplastic and/or thixotropic response, steric hindrance and/or electrostatic repulsion of the pigments.
Additives which bring about thixotropy are described in EP-0 198 519 and DE-A-18 05 693. DE 39 22 178 discloses that mixing a suspension of a flaky substrate with spherical particles such as SiO.sub.2, TiO.sub.2 and ZrO.sub.2 gives deagglomerated and readily dispersible pigments.
However, these additives can have a negative influence on the quality of the coating. Especially the brilliance of pearl luster (or "nacreous") pigments and the uniformity of the coating can be impaired. In general, the severity of these impairments increases with the concentration of the additives used.